1. Field of the Invention
The present invention relates to a method utilized in a wireless communication scheme and related communication device, and more particularly, to a method and related communication device utilized in a wireless communication system for improving a random access procedure associated with a time alignment timer.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile devices, also referred as user equipments (UEs).
Architecture of the radio interface protocol of the LTE system includes three layers: the Physical Layer (L1), the Data Link Layer (L2), and the Network Layer (L3), wherein a control plane of L3 is a Radio Resource Control (RRC) layer, and L2 is further divided into a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer and a Medium Access Control (MAC) layer.
In the LTE system, if a mobile device such as a mobile phone desires to connect to the Internet or communicate with other mobile phones via the LTE system, the mobile device firstly needs to be synchronized with a base station that serves the mobile device on uplink (UL) timing. The purpose of being synchronized with the base station is to prevent signals transmitted from the mobile device from colliding with other signals sent from other mobile devices under the coverage of the base station. In general, a time alignment timer of the mobile device is utilized for indicating whether the mobile device is synchronized with the base station on uplink timing. When the time alignment timer is running, uplink timing synchronization is still established. If the time alignment timer expires, then this indicates that the mobile device is not synchronized with the base station on uplink timing.
FIG. 1 is a diagram showing a Random Access (RA) procedure of the LTE system according to the prior art. As can be seen from FIG. 1, when a user equipment (UE) 210 initials an RA procedure, an RA preamble is transmitted from the UE 210 to the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) 220. The E-UTRAN 220 needs to transmit an RA response (RAR) corresponding to the RA preamble to the UE 210. After that, the UE 210 transmits a media access control protocol data unit (MAC PDU) for contention resolution to the E-UTRAN 220, wherein the MAC PDU usually consists of a MAC header and zero, one or more MAC Control Elements (CE). A MAC RAR usually consists of three fields: TA (Timing Advance)/UL Grant/Temporary C-RNTI.
However, it is not clear how to handle an ongoing RA procedure when a Time Alignment Timer expires. More specifically, when a Time Alignment Timer expires during an ongoing RA procedure, how to handle the unfinished ongoing RA procedure is not specified in the 3GPP specification. In addition, the 3GPP specification defines that when a Time Alignment Timer expires in a UE, the UE flushes all HARQ buffers. This makes a failure of a retransmission for a MAC PDU transmission for contention resolution when there is a HARQ NACK from the eNB, and the failure of the MAC PDU retransmission can even hang the UE, as shown in FIG. 2. In FIG. 2, a UE under RRC-CONNECTED mode with UL sync but without a UL grant is trying to initiate a UL data transmission by sending a RA preamble first to an eNB.
When a downlink (DL) data arrival occurs, the eNB can use a PDCCH (physical downlink control channel) order to request the UE to perform a RA procedure if the eNB considers that the UE no longer has uplink synchronization. Please refer to FIG. 3, where a PDCCH order for downlink data arrival is received by a UE under RRC_CONNECTED mode and an uplink transmission is initiated by the UE whose uplink timing is still synchronized. When the uplink timing of the UE is still synchronized, a time alignment timer is in a running state. Since the time alignment timer is running, PUCCH (physical uplink control channel) resource is considered available. In one case, the UE sends a scheduling request (SR) on PUCCH, but the eNB discards the SR and does not send a RAR to the UE since the uplink synchronization of the UE is considered lost. As a result, sending a SR is useless and it is wasteful of UE power in such case. After this, the UE may transmit a SRS (Sounding Reference Symbol) or a CQI (Channel Quality Indicator) according to RRC configuration. In another case, the UE fails to receive the RAR sent from the eNB and consequently triggers a SRS or a CQI to the eNB. However, without the timing advance information from the RAR, the eNB may fail to receive the SRS and CQI in both cases since the timing advance is not updated to an accurate value.
A NDI (New Data Indicator) is used for indicating whether the corresponding transmission is a new transmission or a retransmission. Each NDI is compared with the previous NDI. Please refer to FIG. 4, whereas UE under RRC_CONNECTED mode with a running time alignment timer handles a UL transmission after receiving a PDCCH order for DL data arrival. Since the time alignment timer keeps running through the steps of FIG. 4, the UE does not flush HARQ buffers and does not consider the next transmission for each process as the very first transmission. The very first transmission is a transmission with no available previous NDI. However, the NDI on PDCCH is randomly selected by the eNB. This impacts the following uplink transmission because the UE may find that the NDI on PDCCH is identical to the previous NDI and thereby performs a retransmission of data in one HARQ buffer. Any transmission after such retransmission should be a new transmission instead of a retransmission.